LeetCode - Product of Array Except Self

Given an array nums of n integers where n > 1,  return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].

Example:

Input:  [1,2,3,4]
Output: [24,12,8,6]
Note: Please solve it without division and in O(n).

Follow up:
Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)

class Solution {
    public int[] productExceptSelf(int[] nums) {
        if(nums == null || nums.length == 0){
            return null;
        }
        int[] res = new int[nums.length];
        int right = 1;
        
        //before the element
        for(int i = 0; i < nums.length ; i++){
            if(i == 0){
                res[i] = 1;
            }
            else{
                res[i] = res[i-1] * nums[i-1];
            }
        }
        
        //after the element
        for(int i = nums.length - 1; i >= 0; i--){
            res[i] *=right;
            right *= nums[i];
        } 
        return res;
    }
}